An electric double-layer capacitor (EDLC), also known as a “supercapacitor” or “ultracapacitor”, is a type of electrochemical capacitor, which is characterized by a very high energy density relative to conventional capacitors. Instead of two metal plates separated by a regular dielectric material, an EDLC involves the separation of charges in a double electric field formed at the interface between an electrolyte and a high surface area conductor. A basic EDLC cell configuration is a pair of highly porous electrodes, typically including activated carbon, disposed on opposite faces of parallel conductive plates known as current collectors. The electrodes are impregnated with an electrolyte, and separated by a separator consisting of a porous electrically-insulating and ion-permeable membrane. When a voltage is applied between the electrodes, negative ions from the electrolyte flow to the positive electrode while positive ions from the electrolyte flow to the negative electrode, such that an electric double layer is formed at each electrode/electrolyte interface by the accumulated ionic charges. As a result, energy is stored by the separation of positive and negative charges at each interface. The separator prevents electrical contact between the conductive electrodes but allows the exchange of ions. When the EDLC is discharged, such as by powering an external electrical device, the voltage across the electrodes results in current flow as the ions discharge from the electrode surfaces. The EDLC may be recharged and discharged again over multiple charge cycles.
The extremely high surface area of the activated carbon electrodes, combined with a separation distance between electric double layers on the order of nanometers (compared with millimeters for electrostatic capacitors and micrometers for electrolytic capacitors), enables the absorption of a large number of ions per unit mass and, thus, an energy density that is orders of magnitude greater than that of conventional capacitors. The electrolyte may be an aqueous-based solution (e.g., a water solution of potassium hydroxide (KOH) or sulfuric acid (H2SO4)) or organic-based (e.g., acetonitrile (CH3CN), polypropylene carbonate). In an aqueous-based electrolyte, the voltage is limited to approximately 1V (above which water decomposes), whereas organic-based electrolytes have a higher maximum voltage of about 2.5-3.0V. Since each individual EDLC cell is limited to a relatively low voltage, multiple EDLC cells may be connected in series to enable higher voltage operation. However, serial connection reduces the total capacitance and also requires voltage-balancing.
While the amount of energy stored per unit weight is generally lower in an EDLC in comparison to electrochemical batteries, the EDLC has a much greater power density and a high charge/discharge rate. Furthermore, an EDLC has a far longer lifespan than a battery and can undergo many more charge cycles with little degradation (millions of charge cycles, compared to hundreds for common rechargeable batteries). Consequently, EDLCs are ideal for applications that require frequent and rapid power delivery, such as hybrid vehicles that are constantly braking and accelerating, while batteries are used to supply a larger amount of energy over a longer period of time. EDLCs are also environmentally friendly (have a long lifespan and are recyclable), safe (no corrosive electrolytes and other toxic materials requiring safe disposal), lightweight, and have a very low internal resistance (ESR). The charging process of an EDLC is also relative simple, as it draws only is the required amount and is not subject to overcharging. An EDLC has a higher self-discharge compared to other capacitors and electrochemical batteries.
During EDLC operation at high operating temperatures and/or high operating voltages, various potentially detrimental parasitic effects tend to occur. In particular, electrochemical reactions cause excessive pressures in the electrode composition, resulting in the discharge of gases. The built up pressures from the discharged gases could result in swelling or bursting of the capacitor elements.
Advances in materials and manufacturing methods in recent years have led to improved performance and lower cost of EDLCs, and to their utilization in various applications. For example, EDLCs can be employed to operate low-power electrical equipment, and to provide peak-load enhancement for hybrid or fuel-cell vehicles. EDLCs are also commonly used to complement batteries, such as in order to bridge short power interruptions in an uninterruptible power supply.
U.S. Pat. No. 4,697,224 to Watanabe et al, entitled “Electric double layer capacitor”, is directed to an EDLC which includes an electrically insulative and ion-permeable separator, and a pair of polarizable electrodes of solid carbonaceous material which are disposed opposite each other on opposite sides of the separator. The separator and electrodes are sealed within a gasket of insulating rubber. The separator and at least one of the electrodes are adhered to each other by an adhesive or cohesive agent in part of a region in which the electrode faces the separator, in order to prevent possible displacement of the electrodes and shorting via mutual contact.
European Patent No. 786,786 to Varakin, entitled “Capacitor with a double electrical layer”, discloses an EDLC with one electrode made of nickel oxide and the other electrode made of a fibrous carbonic material, preferably nickel-plated or copper-plated. The electrolyte is an aqueous solution of an alkali metal carbonate or hydroxide.
U.S. Pat. No. 6,201,685 to Jerabek et al, entitled “Ultracapacitor current collector”, discloses a nonaqueous ultracapacitor with current collectors comprising a conductive metal substrate, such as aluminum, which is coated with a nitride, carbide or boride of a refractory metal. The coating is intended to prevent the formation and thickening of a highly resistive aluminum oxide layer on the current collector.
U.S. Pat. No. 6,594,138 to Belyakov et al, entitled “Electrochemical capacitor and method for making the same”, is directed to an electrochemical capacitor with a bank of elements made up of series-connected internal elements and end elements. Each internal element includes an electron-conducting collector, porous different-polarity electrodes disposed on opposite sides of the collector, and electron-insulating separators mounted on the electrodes. Each end element includes a collector and an electrode of an appropriate polarity disposed on one of its sides. The electrodes and separators are impregnated with an electrolyte. The solid phase-to-liquid ratios of the electrodes are selected to lower the probability of electrolyte leakage during assembly and to minimize internal resistance of the capacitor. The capacitor body includes interconnected hold-downs with electron conductors for levelling-out the voltage in the series-connected elements. A polymeric coating is applied onto the conductors, to prevent short-circuiting of nearby elements to the electrolyte. The bank is also coated with a polymeric composition for sealing the elements, where the coating includes an additional layer that eliminates the effect of the neutralizing component on the rate of polymeric hardening. The bank is evacuated at a residual pressure of 9.8-19.6 kPa prior to mounting between the hold-downs, enabling removal of excess air dissolved in the electrolyte during colloidal milling of the electrode mass.
U.S. Pat. No. 6,773,468 to Lang, entitled “Method of making electrochemical capacitor using a printable composition”, is directed to a preparation method for an electrochemical capacitor cell that includes: a pair of current collector plates placed in parallel; flat electrodes containing aqueous electrolyte printed on opposing faces of the current collectors; and a separator intersposed between the electrodes. The electrodes are printed such that a peripheral region not covered by the electrode is defined on each of the faces of the current collectors. The geometric form and size of the separator is identical to the form and size of the current collector plates. The separator includes a central region permeable to the electrolyte surrounded by a peripheral masked region non-permeable to the electrolyte, where the permeable region coincides with the electrodes. A sealant is impregnated in the pores in the peripheral region of the separator. At least one layer of adhesive is deposited on the sealant. The electrodes are fabricated using a suitable printable composition.